High stability mode-locked lasers are of importance for research and industrial applications, among others. In order for such a laser to be mode-locked, the cavity length of the laser must be matched to the frequency of the loss modulator (e.g., an acousto-optic modulator) of the laser. The loss modulation frequency of the laser corresponds to the pulse repetition frequency. This matched condition can be expressed as follows:f=c/(2*L),   (Eq. 1)where f is the loss modulation frequency of the mode-lock pulse train, c is the speed of light, and L is the optical path length of the laser cavity. Usually, thermal effects on the laser components and optical base are the primary cause of laser cavity length variations. In such case, the mode-lock laser can become unstable. Either the optical path length of the laser cavity or the frequency of the loss modulator has to be adjusted to keep them matched in accordance with Eq. 1
Several methods of stabilizing mode-lock lasers are known. A first approach utilizes a simple feedback loop, which detects the mode-locked pulses and then uses the amplified detector output to drive the laser mode-locker. A second known method utilizes a phase-lock loop, which compares the detected output of the laser to the signal applied to the mode-locker, with the resultant error signal being used to correct the drive of the mode-locker.
Both of the aforementioned methods have similar disadvantages, however, as neither can easily be used with a loss modulator mode-locking element since time varying losses inside the cavity introduce relaxation oscillation noise on the laser output which, in turn, produces noise in the feedback loops. When a loss modulator is driven hard enough to produce short mode-locked pulses, such noise can increase to a level that causes these feedback systems to lose lock. Both known methods also allow the transmitter frequency to vary continuously. (See U.S. Pat. No. 4,025,875).
In a third known mode-locked laser stabilization method, a portion of the fundamental optical radiation is used to generate second harmonic radiation for the purpose of detection. The amount of the power of the second harmonic frequency depends on the match between the optical length of the laser cavity and the mode-lock frequency. In this method, a significant portion of the power of the mode-locked laser is used just for detection. In addition, an expensive nonlinear crystal is needed, and a dither frequency is present on the laser output, which may be undesirable for some applications.